Pancreatic cancer is the fourth leading cause of cancer death among adults in the United States (1, 2). In the year 2005 alone, an estimated 32,180 new cases of pancreas cancer were predicted in the United States and 31,800 patients with pancreas cancer were expected to die (2). The five-year survival rate is about 4% (1, 2). This is largely due to the lack of symptoms and diagnostic tools for the detection of this disease at early stages as well as a deficiency of effective therapeutics for later stage disease.
In addition, pancreatic cancer is a highly chemoresistant malignancy. The most effective first-line drug for patients with advanced pancreatic cancer is gemcitabine, which provides only a very moderate improvement in survival (3). Thus, the discovery of new treatment for patients with pancreatic cancer is critically important.
One of the main genetic alterations in pancreatic cancer is the loss-of-function mutation of the DPC4 (Deleted in Pancreatic Cancer locus 4) tumor suppressor gene. The DPC4 gene is located on chromosome 18q21, a region that is homozygously deleted in 30˜37% of pancreas ductal adenocarcinomas (4, 5). Intragenic inactivating mutations such as nonsense, misssense, and frameshift also occur commonly in the DPC4 gene, accompanied by a loss of the other allele, resulting in the loss of heterozygosity (5, 6). In total (both homozygous deletion and loss of heterozygosity), the DPC4 gene is inactivated in approximately 55% of patients' tumors (1, 7).
The loss of DPC4 gene is thought to be associated with the progression and malignancy of pancreatic cancer, as it occurs only in PanIN3 and pancreas adenocarcinomas (8, 9). DPC4 deficiency is also associated with poor survival of patients with pancreatic cancer. Patients with pancreatic cancer with normal DPC4 expression have significantly longer survival times. The unadjusted median survival was 19.2 months in patients with pancreatic cancers with DPC4 protein expression compared with 14.7 months in patients with pancreatic cancers lacking DPC4 protein (10).
In summary, loss of DPC4 has been considered as a predictor of decreased survival in pancreatic cancer (10, 11). Of additional interest is that loss of DPC4 is associated with progression and malignancy in other types of tumors as well. For example, Kuroki et al. reported that among 176 colorectal tumors at varying stages, DPC4 is lost in 0% (0/40) of adenomas, in 10% (4/39) of intramucosal carcinomas, in 7% (3144) of invasive carcinomas without distant metastasis, in 35% (6/17) of primary invasive carcinomas with distant metastases, and in 31% (11/36) of carcinomas metastasized to the liver or distant lymph nodes (12), which is further supported by Lindberg et al. (13).
With its high frequency in pancreatic cancer, DPC4 deficiency presents a target for therapeutic intervention. The underlying promise of targeting DPC4 deficiency in pancreatic cancer is the hypothesis that molecular targets that are critical to the viability of cancer cells, in combination with DPC4 deficiency, exist in the genome of cancer cells and can be explored using small molecular weight compounds. It has been previously reported that some cancer cells with a specific loss-of-function mutation in tumor suppressor genes can be selectively killed by certain antitumor agents. For example, some myeloma cells with PTEN mutations are remarkably sensitive to CCI-779, a rapamycin analog than those with wild type PTEN (14). Erastin, a quinazoline analog, has been shown to specifically kill transformed tumorigenic human foreskin fibroblast cells expressing SV40 small T antigen and the RAS.sup.v12 oncoprotein (15). Similarly, DNA cross-linking agents such as mitomycin C and cisplatin have higher activity in pancreatic cancer cells with BRCA2 deficiency (16). However, none of the references discussed above mentioned any compounds capable of selectively targeting cancer cells deficient in DPC4 gene.
The present invention describes novel antitumor agents with genotype selectivity against cancer cells exhibiting DPC4 deficiency, methods for screening and identification of antitumor agents that selectively eliminate cancer cells with specific patterns of loss-of-function alterations, and methods for preparation and therapeutic use of such antitumor agents.